1. Field of the Invention
This invention relates generally to a phase change developer for liquid electrophotography, and more particularly to a phase change developer that contains a crystallizable polymeric binder resin and the phase change developer reversibly changes from solid phase to liquid phase at or above 22xc2x0 C.
2. Background of the Art
In electrophotography, a photoreceptor in the form of a plate, sheet, belt, or drum having an electrically insulating photoconductive element on an electrically conductive substrate is imaged by first uniformly electrostatically charging the surface of the photoconductive element, and then exposing the charged surface to a pattern of light. The light exposure selectively dissipates the charge in the illuminated areas, thereby forming a pattern of charged and uncharged areas (i.e., an electrostatic latent image). A liquid or dry developer is then deposited in either the charged or uncharged areas to create a toned image on the surface of the photoconductive element. The resulting visible image can be fixed to the photoreceptor surface or transferred to a surface of a suitable receiving medium such as sheets of material, including, for example, paper, transparency, metal, metal coated substrates, composites and the like. The imaging process can be repeated many times on the reusable photoconductive element.
In some electrophotographic imaging systems, the latent images are formed and developed on top of one another in a common imaging region of the photoreceptor. The latent images can also be formed and developed in multiple passes of the photoreceptor around a continuous transport path (i.e., a multi-pass system). Alternatively, the latent images can be formed and developed in a single pass of the photoreceptor around the continuous transport path. A single-pass system enables the multi-color images to be assembled at extremely high speeds relative to the multi-pass system. At each color development station, color developers are applied to the photoreceptor belt, for example, by electrically biased rotating developer rolls.
Image developing methods can be classified into liquid type developing and dry type developing. The dry type method uses dry developers and the wet type method uses liquid developers.
Dry developers are generally prepared by mixing and dispersing colorant particles and a charge director into a thermoplastic binder resin, followed by milling or micropulverization. The resulted developer particle sizes are generally in the range of about 4 to 10 microns, which size particles are readily carried by air movement. For this reason, if the fine powders of a dry developer are scattered, they pose an environmental problem. However, dry particles provide excellent ease of handling and stability for the developer particles.
On the other hand, liquid developers are prepared by dispersing colorant particles, a charge director, and a binder in an insulating liquid (i.e., a carrier liquid). Liquid developer based imaging systems incorporate features similar to those of dry developer based system. However, liquid developer particles are significantly smaller than dry developer particles. Because of their small particle size, ranging from 3 microns to submicron size, liquid developers are capable of producing very high resolution images.
The major problems of liquid developers are the emission of the liquid carrier from liquid developers to the environment during the drying and transfer process due to inefficient solvent recovery system; the need to dispose the waste liquids; and inconvenience since their handling is difficult and frequent maintenance is required for maintaining stable image formation.
It would be desirable to provide a novel phase change developer which provides the advantages of both the dry and liquid developers. The phase change developer must be stable, easy to be handled, pose no environmental problems such as solvent emission and dry toner spill; and provide high resolution images.
The phase change developer can reversibly change from a solid phase to a liquid phase at its melting point or crystallization temperature. The phase change developer is a solid in storage and before image development. During image development, the phase change developer melts at a temperature above its melting point to form a liquid developer which then undergoes a liquid electrophotographic process to produce toned images.
Some phase change developers for liquid electrophotography have been mentioned in U.S. patents. U.S. Pat. No. 5,229,235 discloses a phase change developer comprising a colorant and an insulating organic material having a melting point not lower than 30xc2x0 C. The organic material is selected from the group of normal paraffins with 19 to 60 carbons, waxes, and crystalline high molecular material. The preferred organic materials are paraffins and waxes.
U.S. Pat. No. 5,783,350 claims a phase change developer comprising a colorant, a thermoplastic resin, and an insulating carrier. The insulating carrier is selected from the group of a branched or linear aliphatic hydrocarbon paraffin or wax, a crystalline polymeric resin having a low molecular weight and a mixture of the foregoing. Among these, particularly preferred is a paraffin consisting primarily of an alkane which has a definite melting point and has a low viscosity after fusion.
U.S. Pat. No. 5,886,067 claims a liquid developer comprising a carrier liquid, a charge director, and an organosol having a (co)polymeric steric stabilizer covalently bonded to a thermoplastic (co)polymeric core and said (co)polymeric steric stabilizer comprises a crystallizing polymeric moiety that independently and reversibly crystallizes at or above 22xc2x0 C.
This invention features a phase change developer that includes:
(a) a carrier having a Kauri-butanol number less than 30; and an organosol comprising a graft (co)polymeric steric stabilizer covalently bonded to a thermoplastic (co)polymeric core that is insoluble in said carrier, and said (co)polymeric steric stabilizer comprises a crystallizing polymeric moiety (e.g., located on a side-chain or main-chain) that independently (that is, this moiety may crystallize even if other moieties in the stabilizer do not crystallize) and reversibly (that is, the moiety, after crystallization, can be rendered amorphous by physical processes) crystallizes at or above 30xc2x0 C., wherein said phase change developer has a melting point, exudation temperature, flow temperature or melt temperature at or above 22xc2x0 C.
The phase change developers of the present invention will be described primarily with respect to electrophotographic office printing; however, it is to be understood that these phase change developers are not so limited in their utility and may also be employed in other imaging processes, other printing processes, or other developer transfer processes, such as high speed printing presses, photocopying apparatus, microfilm reproduction devices, facsimile printing, ink jet printer, instrument recording devices, and the like.